This isn't a bad book. It might even be considered a fun book. The characters are true to themselves, Ace gets to blow stuff up and it makes sense with the plot, the Doctor works well with aliens, and it even has plausible unicorns and centaurs and trolls! But one thing it isn't: a page-turner.
I kept feeling like I'd read other parts of this book or seen them in movies before, and the author while in the writing process had run across them and said "Ooh neat! Let's incorporate this!" Book set in the English countryside with Odd Events Happening and American Werewolf In London comes on the telly? Let's put two American male post-teens in it! Need to get Ace from one place to another and Legend strikes your fancy? No problem! (I do not, by the way, think that this settles whether or not Ace is a virgin.) Need someone sympathetic in a police force? Dirk Gently-analogue to the rescue! We even have a minor nod to Peter Davison's other best-known role in a country vet. Again, it's not badly done, per se, but the deja vu got distracting and I'd have to put the book down while I figured out where I'd seen this before.
I also felt the book was left with too many loose ends. There were too many things the Doctor didn't check on, too many things left hanging far too nastily for me, and some things which were given a far happier ending than I felt appropriate for the story.
But the good did outweigh the bad, in the end. I enjoyed the way the society of Tir-Nan-Og was constructed, and how the sub-societies worked. I did feel like the Doctor was regaining his balance by the end. Ace never just pointlessly wandered off; she always had a motivation that felt right for the character. I just wish the book had moved a little faster.